Voltage controlled oscillators (VCOs) are used in various applications, including phase lock loops (PLLs). An example of a conventional PLL 100 that uses a VCO is shown in FIG. 1. The PLL 100 includes a VCO 102, a loop filter 104, a phase comparator 106, a reference frequency oscillator and divider 108, and a programmable counter 110. These components work together to lock onto the frequency of an input signal. In order to do so, the VCO is tuned to match in phase with the frequency of the input signal.
There are various methods to tune the frequency of a VCO. FIG. 2a illustrates a conventional analog method of tuning a VCO by using varactors. This method includes the use of a resonator 204, an amplifier 202, and two varactors 210 and 212 controlled by analog voltage controls Vcontrol 206 and Vcontrol 208, respectively. The capacitance of a varactor varies with the voltage applied across it. Therefore, the analog voltage control will change the value of the varactor capacitance, which changes the frequency of the oscillator. The tuning range of a circuit employing this analog tuning method is limited by the linear range of the varactor capacitance vs. voltage (C-V) curve, which is limited. Achieving a wide tuning range requires that the C-V transfer gain of the VCO increases, which results in higher phase noise. Also, in complementary metal oxide semiconductor (CMOS) processes, special varactors are often not available, forcing the use of MOS for the varactor, which makes the tuning voltage range even smaller.
FIG. 2b illustrates a conventional digital method of tuning a voltage controlled oscillator using a large binary weighted capacitance array. This method includes the use of a resonator 220, an amplifier 218, an analog to digital (A/D) converter 214, and binary weighted capacitors (C, 2C, . . . , 2nC) controlled by a voltage control 216. This method achieves frequency tuning by converting the analog voltage signal into the digital domain using A/D converter 214 and then using the digital signals to switch in or out the binary weighted capacitors. For a wider tuning range, the resolution required for the A/D converter increases, which in turn increases the cost and complexity of the circuit. Also, for a wide tuning range, the capacitance ratio of the highest value to lowest value increases, which makes it harder to realize physically due to stray capacitances.